Meteor Detection by Amateur RadioJuly 1947 QST Article

Wax nostalgic about and learn from the history of early electronics. See articles
from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

The 1940s and 1950s was an era of much advancement in our knowledge
of Earth's upper atmosphere and its affects on radio communications
- both good and bad. Industry, government, academic, and amateur
groups all played major roles in conducting experiments and
publishing findings for the interested community to share and
build upon. A year ago I posted an article, along with a bit
of editorialization, from the July 1958 edition of Radio-Electronics
titled "Communications
Via Meteor Burst."

Meteor Detection by Amateur Radio

A New Field of Observation

By Oswald G. Villard, Jr.,* W6QYT

An interesting and little-known portion of the rapidly-expanding
body of knowledge now being assembled on the ionosphere is the
subject of meteors and their effect on radio propagation. In
a previous article,1 the author told how Doppler
whistles, caused by meteors entering the ionosphere, can be
heard on the signals of high-power short-wave broadcasting stations.
The purpose of this account is to describe a method of hearing
meteor whistles and other effects on the signals of ordinary
amateur stations, using straightforward receiving techniques.
An amateur station can, in fact, be used for "counting" meteors
automatically, with a sensitivity far greater than that of the
human eye. That meteors can thus be painlessly "counted," when
the sky is overcast or bright with daylight, should be of considerable
interest to astronomers as well as to radio engineers concerned
with the behavior of the ionosphere, for the new technique of
meteor detection by radio promises to yield valuable information
in both fields of knowledge. Meteor spotting therefore provides
the inquisitively-minded amateur with an interesting opportunity
to put his hobby to use in gathering worthwhile scientific information.

You're combing through the DX bands when suddenly there's a
strong burst of signal which rapidly dies down into the background.
Where did it come from, and why? The "why" was probably a meteor
trail; this article tells how to use these signals from shooting
stars for systematic meteor observation, night or day, good
weather or bad.

Meteors are much in the spotlight of public attention these
days, because the V-2 and similar rockets are rapidly encroaching
on that domain of the upper air once inhabited exclusively by
shooting stars and fireballs. An important question is: what
happens when these two different manifestations of matter collide?
Will the embryo space ship be completely destroyed, or only
punctured like a partridge riddled with buckshot?

These and similar interesting speculations are left to the
Jules Vernes of our time. As far as the present discussion is
concerned, it suffices that radio has been shown to provide
a new tool for the study of meteors. In view of cosmic hiss
and solar static, radio equipment may some day be as commonplace
a piece of astronomical gear as the telescope.

Some Facts about Meteors

Meteors are, in a very real sense, the driftwood of outer
space. They are simply particles of matter - the rubble left
over, perhaps, when our solar system was constructed. The particles
are graded as to size: the very largest, fortunately, are quite
rare; yet the very smallest are so numerous that counting them
strains the imagination. Particles large enough to survive the
plunge through our atmosphere are called meteorites. There is
a crater in Arizona one mile in diameter and 600 feet deep,
caused in prehistoric times by the impact of one of these visitors
hurtling in from outer space.

The meteors one sees ordinarily are astonishingly small in
size - perhaps as big as a pea. On an average night the casual
observer will see between two and eight meteors of this size
per hour. If a count like this could be maintained over the
entire surface of the earth for a period of twenty-four hours,
the grand total would be about 24 million meteors. If all the
meteors of all sizes which strike the earth every 24 hours could
be counted up, the total would come out to be some eight billion
meteors!

The most remarkable thing about meteors is their speed. We
must think of the brilliant flash of a falling star as being
caused by an object hurtling through space at something like
25 miles per second, or about 50 times as fast as the V-2. It
is small wonder that when these tiny pellets of cosmic dust
collide with particles of our atmosphere, a violent reaction
ensues. Those air molecules unfortunate enough to find themselves
in the path of a meteor are given a tremendous acceleration
by the impact. Glancing off at various angles, they in turn
collide with other molecules, and so forth. The resulting agitation
is not unlike that produced by the passage of electricity through
the rarified upper air, and the result is a visible glow similar
to that of the gas in a neon sign. The same ionization that
produces the glowing streak, or tail, of the meteor, can also
reflect radio waves. Another example of a visible glow produced
by ionization is the aurora borealis, which is caused by a mechanism
as yet not too clearly understood. Six-meter enthusiasts who
have made DX contacts by pointing their beam arrays directly
at the aurora, thus bouncing signals back from its sides, have
taken advantage of the reflective properties of an ionized region.

Most meteors are distributed more or less uniformly in space
and appear at random intervals from random directions. There
are certain times of the year, however, when these sporadic
meteors are supplemented by clouds of cosmic dust particles
all traveling in the same direction, which produce displays
called meteor showers. The shower meteors are bits of matter
sloughed off by comets, and they follow along the same path
as the parent comets even though the comets themselves may have
long since burned out or disappeared.

Whistles & Bursts

The effect of meteors on radio propagation has been speculated
upon and studied for many years. Ionosphere investigators in
19332 found a change in the over-all level of E-layer
ionization during a meteor shower. Later, sudden unexpected
rises and dips found in charts of radio field strength were
traced to meteors.3 In 1941 reflections from meteor
trails were detected on ionosphere echo-sounding records.4
Not long thereafter, the Doppler whistles caused by the motion
of the meteor trails were discovered,5 and during
the war 100-megacycle radar echoes from meteor ionization were
identified and reported.6 Moreover bursts of signal
received beyond the normal range of f.m. stations were connected
with meteors.7

Recently, however, research in this field has gone ahead
rapidly. During the Giacobini-Zinner meteor shower of October
9, 1946, the wartime radar detection of meteors was duplicated
with great success,8 while at both Stanford University9
and at Harvard University10 meteors were detected
by their Doppler whistles as well. If there had been any doubts
up to that time that meteors were the cause of the effects previously
noted, they were removed on what meteor investigators will remember
as "G-Z day."

Each meteor, entering the ionosphere about 50 or 60 miles
up, produces a thin cylinder of very intense ionization until
it is burned out or dissipated. Oddly enough, only some 10 per
cent of the total energy in a meteor is wasted in friction;
the remaining 90 per cent is spent in producing ionization.
Moreover, the speed of a meteor changes very little (perhaps
10 per cent) during its brief life, and its course is, for all
practical purposes, a straight line.

It is conjectured that the intense ionization contained in
the thin cylinder rapidly diffuses outward, thus increasing
the diameter of the ionized region, However the intensity of
the ionization contained in the cylinder is thereby decreased,
and its level soon drops below that required to reflect a radio
wave of given frequency. At any frequency, then, the strongest
reflection from a meteor trail will be obtained when the dimensions
of that trail are such that the largest volume of ionization
is present of an intensity sufficient to reflect that frequency.
If a very high operating frequency is picked, the cylinder of
ionization is capable of reflecting a signal only when it is
relatively small in diameter, for only then does it have sufficient
intensity. And a small cylinder will return only a weak signal
because of its small "echoing area," When the radio frequency
is up in the several hundreds of megacycles, the size of a cylinder
capable of reflecting those frequencies is so small that only
a very feeble reflection can be obtained. Consequently meteor
reflections have not so far been noted on frequencies much above
100 megacycles. On the other hand, when lower frequencies are
used, the cylinder of ionization can become quite large and
still reflect a signal.

The energy reflected from meteor trails produces two different
types of effects noticeable at the receiver when c.w. signals
are used. The mechanism involved is illustrated in Fig. 1. It
is assumed in this sketch that the transmitting and receiving
aerials are so located and orientated that direct signal from
the transmitter is reduced to a very low value (of the order
of a few microvolts) at the receiver. The frequency must be
high enough so that no reflection from the ionosphere directly
overhead is obtained. Moreover it should be so high that no
"long-scatter" signals are returned from points some distance
away. In fact, the ideal frequency to use is one just high enough
so that no long-distance transmission in any direction is possible,
since "long scatter" cannot then exist. Under these conditions
it will be readily possible to hear bursts of signal reflected
from the sides of the meteor trails, as well as Doppler tones
produced by signals scattered from the moving head of each trail.
Energy scattered back from the moving head of the ionization
columns arrives at the receiver via a path of rapidly-changing
length. This path-length change causes an apparent shift in
the frequency of the reflected signal, which in turn gives rise
to an audible beat note when the reflected signal is combined
with energy reaching the receiver via a path of unchanging length.

Fig. 1 - The basic geometry of meteor detection.

Once the column of ionization has become fully formed, the
signal reflected from it traverses a path of constant length
so that no Doppler shift, and hence no beat note, occurs. As
far as the receiver can tell, the signal from the transmitter
has then suddenly increased in strength; this sudden increase
is called a "burst." The burst will be strongest when the position
of the column of ionization is such that a line can be drawn
from the transmitter and receiver perpendicular to the cylinder
at some point. There results a broadside reflection that will
be much stronger than the signal scattered back when the column
is so positioned that this cannot occur. (It should be remembered
that the E region of the ionosphere, where meteor ionization
occurs, is a relatively thin layer.) The effect is analogous
to the flash of light when sunlight is reflected by a mirror
into the eyes of an observer. The mirror can be seen at all
times - which is to say it reflects back some light - but only
when it is correctly orientated does it produce a flash.

Of the two effects - "Doppler whistles" and "bursts" - the
latter are, of course, much more easily detected, because the
broadside reflection is so much stronger than the scattered
energy returned from the moving head of the average meteor trail.
However, the strength of the "whistle" is not so dependent on
the orientation of the meteor's path. The head of the column
of ionization, being small and of rounded shape, apparently
scatters signals back well in many directions. Consequently
most meteors that are large enough will produce a whistle. However,
only those meteors which travel along exactly the proper path
will produce a pronounced burst.

This difference in behavior of "whistles" and "bursts" is
readily noticed in practice. Using the set-up described in this
article, one often hears the telltale whistle of a meteor boring
into the ionosphere, without any perceptible change in received
field strength - or "burst" - whatever. These whistles correspond
to the meteors that caromed off in such a way that no broadside
reflection could occur. Then again, one notices "bursts" without
hearing any accompanying whistle. These "bursts," it is reasoned,
are produced by meteors following the correct path for broadside
reflection, but so small that the energy scattered from their
moving heads cannot be heard. The most dramatic-sounding meteors
of all are those that begin with a high pianissimo whistle and
end with a low, fortissimo grunt, or "burst." Here the "burst"
occurs when the meteor trail passes the point at which it can
return a broadside reflection. Interestingly enough, this point
is also the point at which the Doppler beat note goes to zero,
since when the meteor is moving along a path perpendicular to
a line drawn between the meteor and the transmitter and receiver,
there is no change in path length and hence no Doppler shift
of the radio frequency.

It has been found that the ratio of whistles to bursts is
rather sharply dependent on the operating wavelength. If one
listens to short-wave broadcasting stations in the 25- or 31-meter
bands, one hears a relatively large number of whistles as compared
to the bursts, although the latter are somewhat obscured by
the variable "long-scatter" signal always present. Many of the
whistles last as long as one or two seconds. In the 27-Mc. band,
however, the whistles are less frequent than the bursts and
they are of shorter duration. It is likely that at still higher
frequencies (say 100 Mc. or so) whistles would be heard much
less frequently, if at all. The bursts also become of shorter
duration as the frequency is increased. At 27 megacycles, the
average burst is about half a second in duration. At 50 -megacycles,
they appear to be still shorter. When the receiver beat oscillator
is off, bursts sound like a "thump"; with the beat oscillator
on, they sound like a sharp "ping."

Detecting Meteors

The experimental set-up used at Stanford University to detect
meteors is extremely simple. The radiated signal is provided
by the Stanford Radio Club's transmitter, W6YX. Input to the
final stage is 950 watts. Two types of transmitting antennas
have been successfully used: the first is a simple half-wave
doublet roughly 16 feet long and 8 feet above the ground, giving
a radiation pattern consisting of a broad lobe pointed straight
up. The doublet was supplanted by the arrangement shown in the
photograph; This is nothing more than a three-element beam so
arranged that it can be directed vertically upward, or to any
intermediate angle, by means of a rope and pulley. A rotatable
transmitting antenna is a great help in reducing the signal
radiated in the direction of the receiver, since the null off
the ends of the elements can be found experimentally and pointed
in the direction of the receiving site. This null may or may
not be exactly aligned with the direction of the elements, depending
on whether the system is exactly balanced to ground or not.

Directivity is not necessarily an advantage in the transmitting
antenna, since power gain is obtained by decreasing the width
of the beam, which cuts down the area of the sky from which
meteor reflections can be obtained. The practical effect is
to make the number of echoes heard less frequent. Those which
are heard, however, are stronger.

The receiving location at Stanford is an experimental building
about a mile away from the amateur transmitter and somewhat
below the direct line of sight. The receiving equipment is shown
in the second photograph. For purposes of illustration, the
NC-200 receiver is shown outside the building - it is normally
located inside where there is a heater! A ten-foot post, set
in the ground, supports the 16-foot 2-by-4 on which the 11-meter
dipole antenna is mounted.

The unique feature of this antenna is that it not only can
be turned in any desired direction but can be tilted at will.
It is connected to the supporting post by what is in effect
a swivel joint. Tilting and turning is accomplished by means
of fish lines tied to the ends of the antenna; an awkward method,
but one that works! The object is to find that position of the
receiving antenna at which the direct signal from the transmitter
is almost completely balanced out. Just why this antenna must
usually be tilted in order to find the null is not very thoroughly
understood; it is conjectured that local distortions of the
field by reradiation from adjacent antennas, power lines, etc.,
gives the incoming wave a polarization that is far from horizontal.
It is commonly observed that the apparent direction of arrival
of a signal, under similar circumstances, may be far different
from the true direction. However, this much can be said for
the tilted antenna: in all cases, no matter how bad the unbalance
to ground, or how many the obstructions (such as cars) close
at hand, it has always been possible to find a sharp null by
properly rotating and tilting it. The signal from the transmitter
at the receiving site is about 30 db. above S9 on the NC-200
S-meter when a nondirectional antenna is used; using the dipole,
this signal can be reduced in strength until it drops into the
noise level.

Something different in beam antennas: the
W6YX tiltable three-element rotary! The antenna can be pointed
vertically for meteor detection, as well as horizontally for
ham QSOs.

There is no special reason why the receiving and transmitting
antennas were located as close together as they were at Stanford,
except convenience. As a matter of fact, the closer they are
together the more difficult it is to find and maintain a deep
null, and the receiving antenna must often be tilted until it
is far from horizontal. These difficulties can be avoided by
greater separation between transmitter and receiver. If the
separation is great enough it may be possible to do away with
special receiving and transmitting antennas entirely, provided
the aerials available shoot the majority of their power toward
the zenith.

It is important that a sensitive receiver be used for meteor
detection, and that the antenna be properly matched to it. Receiver
sensitivity, in this case, is the equivalent of transmitter
power; with the latter set at the 1-kilowatt maximum, and antenna
directivity restricted, system performance can only be improved
by improving the receiver. The best indication of a receiver's
sensitivity is the change in noise level when the first tuned
circuit is tuned through resonance with the gain control wide
open and with no signal being received. Unless there is a noticeable
change when this is done, the set simply isn't sensitive.

The procedure used in making the tests was to radiate an
unmodulated signal from W6YX, null out this signal at the receiving
site, and then maintain an aural and a visual watch for meteor
reflections. Tests of any duration were made in the 11-meter
band, where AØ operation is permitted. The transmitter
was identified by keying the call letters every ten minutes.

When To Listen

The arrival of a meteor will be announced either by a brief
whistle audible over headphones or loudspeaker, or by a sudden
"kick" of the receiver's S-meter. Often the whistle and kick
will nearly coincide. The pitch of the whistles in most cases
descends rapidly to zero, ending in a "grunt." In some instances
it may go to zero and then start to rise again, showing that
the meteor has approached, passed by at right angles, and then
begun to recede. In most cases, however, the meteor will pass
through the ionized region or will burn itself out before an
"up" whistle can be formed.

Receiver and tilting 27-Mc. dipole antenna
used for receiving meteor whistles. The antenna is rotated and
tilted by means of fish lines until the direct signal from the
transmitter is balanced out .

An oscillogram of a meteor whistle is shown in Fig. 2. This
oscillogram, believed to be one of the first of its kind, was
made by transcribing a phonograph recording of the whistle on
a 16-mm. motion-picture sound track. At the left of the record
(time runs from left to right) will be found random fluctuations
caused by the noise output of the receiver in the absence of
meteor signal. These fluctuations gradually become regular as
the whistle fades in, and the downward change in pitch can readily
be seen. As the whistle pitch goes to zero, the strength of
the reflected signal increases and presently the receiver is
blocked by the "burst" or broadside reflection. The background
noise accordingly disappears. The burst then fades away and
as the set recovers, the noise again puts in its appearance.
During the burst, when the receiver noise is absent, a series
of regularly-spaced marks will be found on the record. These
are 15-c.p.s. timing pulses added to give an idea of the duration
of the burst. /p>

The best hours for hearing meteors, unfortunately, are the
wee small ones early in the morning. This is because the earth,
while performing its daily rotation, is at the same time moving
forward in space along the track of its yearly orbit around
the sun. From midnight on, that tiny speck of the earth's surface
that we call "home," is moving forward in space at a speed equal
to the sum of the motions resulting from the earth's spin and
that of its orbital travel. During the afternoon and early evening
hours the net forward speed of "home" is the difference of these
two motions. The situation is the same as that of a fly clinging
to the rim of a moving wagon wheel, considered with respect
to the road's surface. The fly is moving forward faster when
he is on the top of the wheel than when he is down near the
road. When our portion of the earth is moving forward in space
most rapidly, the ionosphere directly above us runs into the
most meteors, and vice versa.

However, meteors can be "heard" in the late morning and early
evening hours too; they are simply less frequent, and it may
be necessary to wait a longer time to hear one. It has been
found that the fishing is best between the hours of 2 and 4
in the morning. Interestingly enough, these are the best hours
for visual observation as well.

On ordinary nights, the number of bursts heard on 11 meters
with the set-up described above varied between something like
40 or 50 per hour during the early evening hours, up to something
over two hundred per hour during the early morning hours.

The number of whistles heard was roughly one tenth the number
of bursts. (A reduction in transmitted power will not greatly
affect the number of meteors detected. In the course of some
tests with the W6YX buffer-amplifier, whistles and bursts were
plainly heard with a. radiated power of roughly 150 watts. And
no attempt was ever made to improve the performance of the receiver
by adding additional r.f. preamplification!) In the vicinity
of the various meteor showers the number will be considerably
greater. The Lyrid meteor shower. of April 21, 1947, caused
an increase of roughly 3 times in the number of meteors "heard,"
for example.

There follows a tabulation of the nine principal meteor showers
each year, taken from Reference 6:

Name

Duration in Days

Date of Maximum

Quadrantids

3

January 2

Lyrids

4

April 20

Eta cquarids

8

May 2-4

Delta Acquarids

3

July 28

Perseids

35

August 11-12

Orionids

14

October 19-23

Leonids

3

November 14

Andromedes

2

November 24

Gemenids

14

December 11-13

The above dates should be taken as only approximate, as there
is a variation from year to year. The exact dates of each shower
may be obtained in advance by consulting such publications as
Sky and Telescope magazine, which may be found in any public
library.

Fig. 2 - A sound-on-film record of a meteor
whistle and burst. A - random set noise before appearance of
meteor; B - the "whistle," starting with a high pitch at the
left and rapidly descending to zero-beat at the right; C - the
"burst," when the signal strength washes out the receiver noise;
D - the burst tailing off, set noise increasing; E - signal
no longer perceptible and set noise again dominates.

The "ticks" in the burst region (C-D) are time signals at
intervals of 1/15 second, so the major burst signal lasted approximately
1/3 second in this instance.

A great many other things will be heard as well as meteors.
Since the receiver must be wide open, with the direct signal
from the transmitter reduced to so low value that it does not
operate the receiver's a.v.c. or cause any change in gain, there
will be a continuous roar of set noise in the loudspeaker. Needless
to say, a receiving location that is electrically quiet is essential.
Any cars moving in the vicinity will give rise to reflections
which will upset the balance; they usually give rise to a fluttery
motion of the S-meter needle and a low-pitched rumble in the
loudspeaker.

Some Possibilities

It is extraordinary to think that meteor trails, occurring
as they do some 50 or 60 miles from the observer, should be
able to reflect radio signals of about the same strength as
airplanes flying overhead roughly one mile away. Yet, echoes
from strong meteors often kick the NC-200 S-­meter up to S9
or above. The size of the ionized region produced by a meteor
must clearly be large. If an airplane, which can be thought
of as an irregular object approximately 100 feet in diameter,
returns an echo of given strength at a distance of 1 mile, an
irregular object such as a meteor trail must be at least 50
times as large in order to return an echo of equal strength
at 50 miles. This implies that the meteor trail must be roughly
5000 feet or one mile in diameter. Moreover, since the tests
were made at 11 meters, with the transmitter and receiver virtually
at the same place, the ionization contained in this trail must
be intense enough to reflect an 11-meter wave fired directly
at it,. i.e. - at vertical incidence! Presumably if this ionization
is allowed to diffuse outward until it is only strong enough
to reflect, say, a 7-Mc. wave at vertical incidence - which
is a level of ionization commonly encountered in the normal
ionosphere - the diameter of the ionization column would then
be very much larger.

It is, of course, also possible for radio signals to be reflected
from meteor trails at glancing incidence, as might be the case
when transmitter and receiver are a hundred or so miles apart
and a horizontally-traveling meteor passes over the midpoint
of the path. Meteor ionization of given intensity would then
reflect radio signals of very much higher frequency. The 6-meter
band "opening" during the Giacobini-Zinner meteor shower, reported
by E. P. Tilton and others, is an example of this effect. Similarly,
the 144-Mc. reflections observed by G. R. Abell, jr.,11
the momentary 28-.Mc. band openings reported by

B. Henke,12 etc., are in all probability caused
by the same mechanism. It is interesting in this connection
to note that the Federal Communications Commission has recorded
at its monitoring station at Grand Island, Nebraska, bursts
of signal reflected by meteors from an f.m. station in Boston,
Massachusetts, operating in the 42-Mc. band!13 This
1400-mile transmission represents about the maximum possible
distance for one-hop E-layer propagation.

Although there is no doubt that meteors can cause the effects
described in this article, the evidence at hand is not by any
means extensive enough to make it possible to say that they
are the only cause. Cosmic-ray bursts, for example, have been
seriously proposed as a source of momentary radio reflections.
It is furthermore quite possible that whatever mechanism produces
sporadic E could also give rise to signal "bursts" of brief
duration.

To heighten the mystery still further, although a great many
coincidences between visually observed meteors and whistles
or bursts have been obtained at Stanford (and it is really impressive
to see a big meteor go sailing overhead while listening to its
whistle in the loudspeaker!) it has nevertheless been found
that a certain percentage of the meteors fully bright enough
to be heard by radio, and apparently occurring in the correct
portion of the sky, are simply not heard at all.

It is therefore clear that a count of whistles or bursts
cannot yet be relied upon to give an indication of the absolute
number of meteors colliding with our atmosphere; however, as
an indication of the relative number, the method is very sensitive
and consequently holds much promise. The behavior during showers,
as well as the observed nightly maxima between 2 and 4 A.M.
shows that meteors play a very large - if not the sole - part
in the formation of whistles and bursts.

But further observation is needed, and in this field the
radio amateur is in a position to make a .unique and important
contribution to our common knowledge of the ionosphere. Anyone
owning a medium-powered transmitter and a sensitive receiver
can use them to spot meteors on cloudy nights as well as clear.
Reports of meteor ionization effects observed are needed and
would be most useful. The author will be glad to correspond
with anyone interested in this type of work.

Acknowledgment

The experiments described in this article have been carried
out jointly by W. E. Evans, jr., R. A. Helliwell, W6MQG, L.
A. Manning, W6QHJ, and the author. Members of the Stanford Radio
Club who have assisted in various ways include: L. A. Roberts,
W6YWX, R. O. Beaudette, W7FXI, and J. W. Menne, W0LTW.

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